The known treatments of this type comprise two phases:
a first phase which consists in heating the wire and holding it at a temperature above the Ac3 transformation temperature so as to obtain a homogeneous austenite; PA1 a second phase which consists in cooling the wire in order to obtain a fine pearlitic structure. PA1 (a) the wire is cooled from a temperature greater than the Ac3 transformation temperature to a temperature less than the Ac1 transformation temperature; PA1 (b) the pearlitization treatment is then carried out at a temperature below the Ac1 transformation temperature; PA1 (c) this cooling and pearlitization treatment is carried out by passing the wire through at least one tube which contains a gas practically without forced ventilation, the tube being surrounded by a heat-exchange fluid in such a manner that a transfer of heat takes place from the wire through the gas in the tube towards the heat-exchange fluid; PA1 (d) the characteristics of the tube, the wire and the gas are so selected that the following relationships are established, at least upon the cooling preceding the pearlitization: EQU 1.05.ltoreq.R.ltoreq.15 (1) EQU 5.ltoreq.K.ltoreq.10 (2) PA1 (a) it comprises means for cooling the wire from a temperature above the Ac3 transformation temperature to a temperature below the Ac1 transformation temperature; PA1 (b) it comprises means which make it possible to effect the pearlitization treatment at a temperature below the Ac1 transformation temperature; PA1 (d) the characteristics of the tube, the wire and the gas are so selected that the following relationships exist, at least upon the cooling preceding the pearlitization: EQU 1.05.ltoreq.R.ltoreq.15 (1) EQU 5.ltoreq.K.ltoreq.10 (2)
One of the most generally used processes is a heat treatment known as "patenting", which consists of an austenitization of the wire at a temperature of 900.degree. to 1000.degree. C. followed by immersion in a bath of lead or molten salts held at a temperature of 450.degree. to 600.degree. C.
The good results obtained, particularly in the case of heat treatment with lead, are generally attributed to the fact that the very high coefficients of convection which are obtained between the wire and the cooling fluid permit, on the one hand, a rapid cooling of the wire between the Ac3 transformation temperature and a temperature slightly greater than that of the lead and, on the other hand, a limiting of the recalescence during the transformation of the metastable austenite into pearlite, recalescense being an increase in the temperature of the wire due to the fact that the energy contributed by the metallurgical transformation is greater than the energy lost by radiation and convection.
Patenting, unfortunately, involves high costs since the handling of the liquid metals or molten salts requires cumbersome technologies and necessitates cleaning the wire after patenting. Furthermore, lead is very toxic and handling it properly for safety to personnel and the environment is expensive.
French Patent Application No. 86/16705 describes a process for heat treating a carbon steel wire so as to obtain a fine pearlitic structure by regulating the temperature of the wire during the transformation of austenite into pearlite in such a manner that it does not differ by more than 10.degree. C., plus or minus, from a given temperature which is less than the temperature of transformation Ac1 and above the temperature of tne pearlitic nose, this adjustment being obtained by passing an electric current through the wire for a period of time greater than the pearlitization time and effecting a modulated ventilation for a part of this time. This process makes it possible to avoid the use of molten metals or salts and it therefore eliminates the problems of environmentally safe handling and of cleaning the wires while leading to simpler installations of more flexible operation. This process, however, requires the use of compressors or turbines in order to obtain a modulated ventilation, which may lead to relatively high investment and operating expenses. Furthermore, this process can be used on an industrial scale only for wires of relatively small diameter, for instance of at most 3 mm.